Physicists have measured and controlled seemingly forbidden collisions between neutral strontium atoms, a class of antisocial atoms known as fermions which are not supposed to collide when in identical energy states. The advance makes possible a significant boost in the accuracy of atomic clocks based on hundreds or thousands of neutral atoms.
Described in the April 17 issue of the journal Science,* the research was performed at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado (CU) at Boulder.
The finding helps eliminate a significant drawback to clock designs based on ensembles of neutral atoms. The presence of many atoms increases both the precision and signal of a clock based on the oscillations between energy levels, or "ticks," in those atoms. However, uncontrolled interactions between atoms can perturb their internal energy states and shift the number of clock ticks per second, reducing overall accuracy. The new techniques make JILA's atomic clock based on strontium atoms 50 percent more accurate, so that it now would neither gain nor lose 1 second in more than 300 million years. (See "Collaboration Helps Make JILA Strontium Atomic Clock 'Best in Class'.")
"This is one of the most precise measurements of collisional effects in a clock," says NIST/JILA Fellow Jun Ye, whose strontium atomic clock design enables scientists to "peek into very tiny effects."
Fermions, according to the rules of quantum physics, cannot occupy the same energy state and location in space at the same time. Therefore, fermions, such as a collection of identical strontium atoms, are not supposed to collide. However, as Ye and his research group improved the performance of their strontium clock over the past two years, they began to observe small shifts in the frequencies of the clock ticks due to atomic collisions. They discovered that two atoms located some distance apart in the same well are subjected to slight variations in the direction of the laser pulses used to boost the atoms from one energy level to another. As a result, the atoms were excited unevenly. Strontium atoms in different internal states are no longer completely identical, and become distinguishable enough to collide. Understanding the process enabled the researchers to reduce or even eliminate the need for a significant correction in the clock output, thereby increasing accuracy.
Beyond atomic clocks, the high precision of JILA's strontium lattice experimental setup is expected to be useful in other applications requiring exquisite control of atoms, such as quantum computing—potentially ultra-powerful computers based on quantum physics—and simulations to improve understanding of other quantum phenomena such as superconductivity.
The research described in Science was supported by NIST, the National Science Foundation, the Office of Naval Research and the Defense Advanced Research Projects Agency.